Electrolytic capacitors that use tantalum, aluminum, or another metal with a valve action can attain a large capacity with a small size by giving the valve action metal that serves as the anode-side counter electrode the shape of a sintered body, an etching foil, or the like to expand the surfaces of the dielectric, and are therefore widely used in common practice. In addition to being small and having a large capacity and a low equivalent series resistance, solid electrolytic capacitors in which solid electrolytes are used as the electrolytes are, in particular, easy to package on a chip, are suitable for surface mounting, and posses other special characteristics, so these features are essential for miniaturizing, increasing the functionality, and lowering the costs of electronic equipment.
In this type of solid electrolytic capacitor, miniature and large capacity applications commonly have a structure in which an anode foil and cathode foil composed of aluminum or another valve action metal are, with a separator interposed therebetween, wound together to form a capacitor element, the capacitor element is impregnated with a driving electrolytic solution, and the capacitor element is housed in a case composed of synthetic resin or in a case composed of aluminum or another metal and then sealed. Aluminum, as well as tantalum, niobium, titanium, and other metals are used as the anode material, and the same type of metal as the anode material is used as the cathode material.
The 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex and Manganese dioxide are known as the solid electrolytes used in solid electrolytic capacitors, but also available in recent years is technology (Japanese Patent Application Laid-open No. 2-15611) that features polyethylene dioxythiophene (hereinafter referred to as PEDT) or another electroconductive polymer that has a low reaction velocity and excellent adhesion to the oxide film layer of an anodic electrode.
A solid electrolytic capacitor in which a solid electrolyte layer composed of PEDT or another electroconductive polymer is formed on such a wound capacitor element is fabricated in the manner shown in FIG. 5. First, the surface of the anode foil composed of aluminum or another valve action metal is roughened by electrochemical etching in an aqueous chloride solution, a plurality of etching pits are formed, and voltage is thereafter applied to an aqueous solution of ammonium borate or the like to form a dielectric oxide film layer (chemical conversion). In the same manner as the anode foil, the cathode foil is also composed of aluminum or another valve action metal, but the surface thereof is subjected to etching alone.
The anode foil on the surface of which an oxide film layer is formed and the cathode foil on which etching pits alone are formed are wound together via an interposed separator to form a capacitor element. Next, a capacitor element that has been subjected to chemical repair is sprayed with separately discharged 3,4-ethylene dioxythiophene (hereinafter referred to as EDT) or another polymerizable monomer, or is impregnated with a mixed liquid of both, and polymerization reactions are accelerated in the capacitor element to produce a solid electrolyte layer composed of PEDT or another electroconductive polymer. The capacitor element is thereafter encased in a cylindrical outer case with a closed end to fabricate a solid electrolytic capacitor.
In recent years, however, solid electrolytic capacitors as described above have come to be used in on-board equipment in vehicles. The drive voltage for an on-board circuit is ordinarily 12V, and a high withstand voltage of 25V is required in solid electrolytic capacitors. However, when manufacturing such a high withstand voltage product with a conventional manufacturing method such as that described above, there is a drawback in that the rate at which shorting occurs in the aging step is high, and the yield is low.
High-melting lead-free solder has come to be used in recent years due to environmental concerns, and the solder reflow temperature has risen from a range of 200 to 220° C. to a range of 230 to 270° C. However, performing solder reflow under such high temperatures has a drawback in that the withstand voltage is reduced. For this reason, a strong need exists for the development of a solid electrolytic capacitor whose withstand voltage characteristics do not degrade even when high temperature reflow soldering is carried out.
Such problems are not limited to the use of EDT as the polymerizable monomer, and the same drawbacks occur when other thiophene derivatives, pyrrole, aniline, or the like are used.
A first object of the present invention is to provide a solid electrolytic capacitor and a manufacturing method thereof that allow the ESR to be reduced and the electrostatic capacity to be improved.
A second object of the present invention is to provide a solid electrolytic capacitor and a manufacturing method thereof that reduce the ESR and improve the electrostatic capacity and withstand voltage.
A third object of the present invention is to provide a solid electrolytic capacitor and a manufacturing method thereof that can improve the yield when manufacturing high withstand voltage products.
A fourth object of the present invention is to provide a solid electrolytic capacitor and a manufacturing method thereof that can improve withstand voltage and inhibit LC fluctuation after reflow.